1. Field of the Invention
The present invention relates in general to a hydraulic control apparatus for controlling a hydraulically operated automatic transmission of a motor vehicle, and more particularly to a device for determining the completion of a shifting operation of a coupling device of the transmission, in order to electronically control the back pressure of an accumulator for minimizing a shifting shock of the transmission.
2. Discussion of the Prior Art
In the art of controlling an automatic transmission of a motor vehicle, there is known a hydraulic control apparatus which uses a cylinder-piston type accumulator connected in a fluid path leading to a hydraulically operated frictionally coupling device, so that the increase and decrease of the fluid pressure applied to the coupling device are suitably controlled when the fluid is fed and discharged to and from the coupling device. An example of such a hydraulic control apparatus is disclosed in laid-open Publication No. 61-149657 of unexamined Japanese Patent Application.
In the hydraulic control apparatus disclosed in the above-identified publication, a frictional coupling device B-2 shown in FIG. 4, for example, is supplied with a line pressure delivered from a shift valve (not shown). The line pressure is also applied to an accumulator 500 which communicates with the coupling device B-2. When the coupling device B-2 is commanded to perform a shifting or engaging action, a piston 501 of the accumulator 500 is moved upward as the line pressure is applied to the accumulator 500 and the coupling device B-2. As a result, the piston 501 is positioned so that the fluid pressure applied to the coupling device B-2 corresponds to a sum of a biasing force of a spring acting on the piston 501 in the downward direction, and a force based on a pressure in a back pressure chamber 502 of the accumulator 500, which force also acts on the piston 501 in the downward direction. The back pressure in the chamber 502 is controlled by an accumulator control valve 300.
Therefore, the fluid pressure applied to the frictionally coupling device B-2 can be regulated by controlling the pressure applied to the back pressure chamber 502 of the accumulator 500. The amount of a torque transmitted by the coupling device B-2 changes with the fluid pressure applied to the coupling device, and consequently the transmission torque of the coupling device can be regulated by controlling the back pressure of the accumulator 500. In other words, the shifting or engaging action of the coupling device B-2 can be effected with a reduced shifting shock, by suitably controlling the accumulator back pressure.
The optimum amount of transmission torque of a frictionally coupling device for assuring a smooth engaging action of the coupling device varies with the output of an engine to which the transmission is connected. In view of this fact, the accumulator is adapted to receive at its back pressure chamber a pressure which varies with the amount of opening of a throttle valve, which in turn reflects the currently required output of the engine.
Recently, the use of a solenoid-operated control valve whose duty cycle is electronically controlled is proposed to make fine adjustment of the accumulator back pressure for regulating the fluid pressure applied to a frictional coupling device, depending upon various running conditions of the vehicle, which includes the amount of opening of the throttle valve. An example of such a solenoid-operated valve is shown at 300 in FIG. 4 of the above-identified laid-open Publication 61-149657.
In the hydraulic control apparatus using an electronic control device for regulating the accumulator back pressure for a frictionally coupling device of an automatic transmission as described above, the regulation of the accumulator back pressure is unnecessary while the transmission is not in the process of a shifting operation. In other words, it is necessary to terminate the back pressure regulation of the accumulator by deenergizing the solenoid-operated control valve, when the shifting action of the coupling device is terminated or completed. To this end, it is proposed to use a timer for estimating the moment at which the shifting action is completed. This arrangement assures relatively improved life expectancy of the solenoid-operated valve.
In the above arrangement using the timer for estimating the completion of the shifting action, the time to be measured by the timer should be set considerably longer than a time during which the shifting action is normally expected to be terminated. Namely, it is required to prevent the solenoid-operated valve from being deenergized to terminate the regulation of the accumulator back pressure before the shifting action is terminated or while the coupling device is still in the shifting action.
To minimize the shifting shock of the transmission, it is desired that the accumulator back pressure regulated by the solenoid-operated valve or the fluid pressure which is applied to the coupling device and controlled by the back pressure be kept at a level which is as low as possible to permit engagement of the coupling device. If the fluid pressure is kept at such a low level for a long period of time due to the above-indicated setting of the timer, the coupling device is likely to undergo undesirable slipping. In this respect, it is desirable to raise the fluid pressure applied to the coupling device to a safe level, as soon as the shifting action has been terminated.
On the other hand, it is known to determine the commencement or termination of a shifting action of a coupling device, based on a change in the detected rotating speed of a rotary member of the transmission. Upon determination of the shifting action, the regulation of the accumulator back pressure is terminated, and the fluid pressure applied to the coupling device is raised to an optimum to hold the coupling device in the engaged position.
However, the above-indicated arrangement may fail to determine or detect the completion of the shifting action of the coupling device, due to some failure, such as a defective speed sensor for detecting the speed of the rotary member, or electrical discontinuity or short-circuiting between the speed sensor and an electronic controller or computer. In this case, the solenoid-operated control valve for the accumulator back pressure is kept energized for a long time, and the fluid pressure of the coupling device is held at a controlled relatively low level determined by various parameters such as the throttle valve opening as indicated above. Consequently, the coupling device may slip, and the control valve may be deteriorated or its life expectancy is reduced.